Tone modifying circuit



TONE MODIFYING C IRCUIT Original Filed April 18, 1953 E H Q /.0 l I Q 6 I 5 S //z //a .4 B7 1"} v u'u'u "Mum INVENTOR Patented July 27, 1937 V UNITED STATES TONE MODIFYING CIRCUIT Charles T. Jacobs, New Providence Township,

Union County, N. J., assignor to Mieslner 1nventlons, Inc., a corporation of New Jersey Application April 18, 1933, Serial No. 666,673

Renewed May 3, 1937 17 Claims.

This invention relates to systems for the transmission and translation into sound of electric oscillations and more particularly to the control of the tone quality oi. the output sound. While 5 the invention may have its widest application in instruments employing electric oscillations for the original production or creation of music, it is also adapted for more general employment, such as. in electro-acoustic systems for the re- 10 production of sound. This application is a continuation in part of my co-pending application, Serial Number 607,677, filed April 27, 1932.

It is an object of my invention to provide improved means and methods for controllably in- 15 creasing the brilliance of the output sound in systems of the class described. a

It is a further object of my invention to provide improved means and methods for con-' trollably modifying the amplitude ratios between 20 different frequency components of the output sound.

It is another object to provide means and methods whereby certain components of the output sound may be controllably emphasized, and

Figure 2 comprises a group of curves i1lustrating the effects produced by the embodiment of 0 Figural; and

Figures 3 and 4 are schematic diagrams of slightly more elaborate embodiments of my invention.

Reference being had to Figure 1, my invention 45 will be seen applied to a musical instrument of the type wherein the vibrations of tuned strings are translated into electric oscillations and these in turn into sound. Such strings are shown as 6, and hammers it are iilustratively' shown as 50 vibrating means therefor. The strings may be of electrically conductive material, and they may conveniently be electrically connected together, as by the use of metallic chevalets 3 supporting the strings. 55 For translating the vibrations of the strings intoelectric oscillations I show an electrically conductive strip ll oi! small cross-section, which may be adjacent and below each string preterably at a constant, small fractional part of its length from the rear end thereof. This strip may be cemented to the top edge of a supporting strip l3 of insulating material, which may be retained in fixed position in any convenient manner. The conductive strip ii is spaced away from the strlngs'in general just sufilciently to avoid contact therewith under conditions of maximum vibration of the latter.

Conductive strip ii may be electrically connected to the grid of a thermionic vacuum tube i5, whose cathode may be energized in any wellknown manner, and whose anode current may be supplied as from a tap H on high voltage battery or other current source ll. The cathode of tube i5 may be established at a potential higher than that of the negative terminal of battery or source H by the flow oi anode current through condensively by-passed resistor i8; and the grid of tube I5 may be biased to the potential of such negative terminal by high resistance l8.

positive with respect to that of conductive strip H, as by connection of one of the chevalets 3 (with which the strings make contact) to the positive terminal of battery or source H. In the output circuit of tube i5 may be provided further apparatus hereinafter described.

It will be appreciated that between each string 8 and the conductive strip ii a small electrostatic capacity exists, and that these capacities in parallel with each other form a total capacity between such strings and conductive strip. This capacity is charged from battery or source I! through resistance 19, and this charge cannot change rapidly because of the high value of the latter resistance. It now any string 6 be vibrated in a vertical plane, as by striking by its hammer ID, the capacity between it and the conductive strip II will be varied oscillatcrily in accordance with the frequency andwaveform of the point of the string opposite the conductive strip. The total capacity between the strings and the conductive strip II will be likewise varied, though in reduced degree; and by virtue of the relatively constant charge in this capacity the voltage across it will likewise vary. The oscillatory variations in this voltage will be seen to be applied to the grid of tube l5 and therefore to appear as variations in its anode current. Thus in the input and output circuits of tube l5 appear electric oscillations translated from the vibratlonsof The strings may be established at a D. C. potential In the anode circuit of tube i! may be provided any convenient means for coupling the output of this tube to succeeding apparatus, and as an example of such a coupling means I have shown a transformer II, through the primary of which will flow the current oscillations in the output circuit of tube IS. The oscillations thereby induced in the secondary 32 of the transformer may be applied to the tone regulating circuit enclosed in the dotted line I, and hereinafter most particularly described. From this circuit they may be applied to the cascade of apparatus shown as thermionic vacuum tube 30, amplifler 38, potentiometer or volume control 40, further amplifier II and loudspeaker 42, by which it will be understood that they are amplified, controlled inrespect of amplitude, and translated into sound, all in a manner substantially uniform at diiferent frequencies.

In my co-pending application Serial Number 564,305, filed September 22, 1931, I showed cer tain tone control circuits operable to vary the transmission efficiency of portions of the electrical system differentially with frequency, and indicated a plurality of uses thereof in the control of harmonic structure of output tones of musical instruments such as the one herein described. The actual circuits therein shown accomplished the differential variation by differently attenuating components of different frequencies-4. e., their transmission emciency never exceeded unity. I have found that other tone regulating circuits may operate with higher absolute efficiencies; and further that they may approximate over at least a limited band of frequencies the effect of controllably rotating the transmission-frequency characteristic about a mean point or region, from a horizontal position to a position tilted in the usually desired direction-i. e., upward at the higher frequencies and downward at the lower. Such tone regulating circuits I have found particularly effective in the duplication, by an instrument of the type herein shown, of the piano and similar instruments, the output tones of which are notably deficient in strong low frequencies and conversely rich in high frequency components.

Such a preferred tone regulating circuit is the one mentioned above and shown in Figure l as comprising auto-transformer 3l,'with primary I4, and variable resistance 35 in series with the primary end of the auto-transformer. Input leads IOI may connect to the primary I4 and resistance 35 in series, and output leads I02 may connect across the entire auto-transformer and resistance 35 in series. The action of the circuit may be understood by considering that the voltage input divides into drop across primary 34 and drop across resistance 35. The former is stepped up by the auto-transformer before transmission by leads I02, while the latter is transmitted by these leads without step-up. With a high value of resistance 35 the drop across 34 is negligible; the transmission efficiency is therefore practically unity at all frequencies. As the value of resistance I5 is decreased, however, the drop across ll-the portion which will be stepped up-increases, more rapidly the higher the frequency. At the same time the input impedance of the circuit is being reduced, particularly at the lower frequencies, and thus progressively greater attenuation of components of these frequencies takes place.

the points of the strings opposite the conductive I strip.

This composite effect may be most generically expressed by showing the transmission efllciency of the circuit in terms of frequency (id/21'), the resistance value (R) to which the variable I may be adjusted, the impedance (Ru) out of which the circuit works, the inductance value (L) of the primary 34 of the auto-transformer II, and the turnsratio (T) between the entire autotransformer and the primary. In these terms the transmission efilciency is given by This assumes a high or infinite input impedance of tube 30, which assumption is justified at audio frequencies. The value of R for the apparatus shown in Figure 1 is essentially the plate impedance of tube l multiplied by the square of the step-up ratio of the transformer 3|.

While an almost limitless number of combinations of values of the components may produce a like number of frequency-transmission characteristics, I may illustrate a typical choice of component values and show the resulting transmission efficiencies at different frequencies for each of several values of R. Thus I have employed the circuit with R0 effectively about 50,000 ohms, L approximately 2 henries, T equal to 5, and R variable from 5,000 ohms or lower to 500,000 ohms. The following approximate transmission efliciencies are thus obtained.

ohms

assess PE". Qu s nun:

This data is graphically shown as Figure 2, wherein curve A shows transmission emciency at different frequencies with a value of R of 500,000 ohms, curve B the same but with a value of R of 50,000 ohms, and curve C the same but with a value of R of 5,000 ohms.

From the above it will be seen that with a high value of R. the transmission-frequency characteristic is essentially uniform or flat; that with intermediate values of R the efficiency is lower at frequencies below approximately 1000 cycles and higher at frequencies thereabove; and that with low values of R such change of efficiency with frequency is increased or made steeper. Still greater steepness of the change with frequency may be effected at intermediate and low values of R by the increase of T, within limits, and vice versa. Furthermore the respective efllciencies for various values of R. may be made to occur at respectively lower or higher frequencies by increasing or decreasing the value of L, since this and the term or appear in the formula above only as multiples of each other. These and other changes of component values from those I have shown will be suggested, for the production of particular characteristics, by the formula itself. In any event, however, it will be found that variation of the value of R from a high or infinite value to a low value will produce progressively greater accentuation of high, and progressively greater attenuation of low, frequency components. The tone regulating circuit shown may be employed if desired with a fixed resistance ll, having resistance R of such a value that a particulareffeet .on tone quality will be obtained; I have always ,preferred, however, to make the resistance 3! variable.

It will be observed that in the choice of the above recited typical values I have chosen L so that the value of all at low frequencies is small compared to R; and that I have made It variable, from a value small compared to that of-Ro and to that of (014 at highfrequencies, to a value large compared to both said last mentioned values.

The auto-transformer 33 has been illustrated in Figure 1 without shunt capacity, and this conditionhasbcen assumed in the formulaegiven above, As a practical matter, however, any auto-transformer which may be employed will have some distributed capacity. This capacity will resonate with the leakage reactance of the auto-transformer to some particular frequency. If the values of distributed capacity and leakage reactance be properly chosen, as is well understood from conventional transformer design, this resonant frequency may be optionally established above the useful limit of audibility or at any desired value below such limit. The effect of a resonant frequency below such limit is to provide additional accentuation of oscillation, and hence sound, components at and near; such frequency, and to provide a slight increase in duration of such components as they may occur in the oscillations being transmitted through the system. The degree of such accentuation and duration increase is of course largely influenced by the resistance of the auto-transformer winding, as is well understood.

I have found, however, that it is also convenient to control the degree of these effects by regulating the input to the auto-transformer proper. In the circuit which I have shown and described such input regulation is obviously effected by adjustment of the resistance 35e. g., when this is of high resistance it limits the current through and voltage drop across the primary 34 to a negligible value. It will thus be seen that when resistance 35 is of high value and therefore provides transmission of the oscillations by the tone control system with negligible alteration of inter-component ratios, it at the same time suppresses modification of the oscillations in respect of the abovementioned extra accentuation and duration increase in the certain frequency band. Conversely, when resistance 35 is of low value and therefore provides a large increase in the amplitude ratios of high to low frequency components, it at the same time permits the resonant conditions of the auto-transformer cir- 'cuit to work an appreciable extra accentuation and duration increase in the particular frequency band. i?

This correlation of effects I have found very useful musically. When the energy of the oscillations and output sound is predominately located in lower frequency components or more or less evenly distributed over the whole frequency range, the effect of extra duration of certain components is probably undesirable. When, however, the energyof the oscillations and output sound is deliberately made to predominate in higher frequencies, an extra accentuation in this frequency region is helpful and a slight duration increase in resistance and leakage reactance may be externally provided. In this case they may be made variable, so that the frequency region at which the resonant effects occur, and the degree of these effects for any particular value of resistance 35, may be independently adjusted. I have therefore shown in Figure 3, which is basically the identical tone regulating circuit shown in Figure 1, the additional elements comprisingvariable resistance iii, variable small capacity 2; and variable inductance i I3. The circuit comprising these elements may if desired be shunted across a portion only of the auto-transformer, as I have shown in Figure 4.

While I have shown and described my invention in, terms of particular embodiments, it will be understood that these are exemplary rather than. comprehensive, and that the scope of my invention is intended to be expressed by the following claims.

I claim:--

1. In a :system including a source of pluralfrequency sound-representing electric oscillations and means responsive to said oscillations: an os dilation-transmitting circuit interposed between said sourse and said responsive means; and adjustable means included in said circuit for modifying the transmission-frequency characteristic of said circuit in a manner substantially corresponding to rotation thereof about an intermediate frequency region thereon.

2. In a system including a source of pluralfrequency sound-representing electric oscillations and means responsive to said oscillations: an oscillation-transmitting circuit interposed be- I tween said source and said responsive means;

and adjustable means included in said circuit.

for modifying the transmission-frequency characteristic of said circuit in a manner substantially corresponding to rotation thereof about a substantially constantly. maintained intermediate frequency point thereon.

3. In a system including a source of pluralfrequency sound representing electric oscillations and means responsive to said oscillations: a circuit having normally a substantially flat transmission-frequency characteristic interposed between said source and said responsive means; and adjustable means included in said circuit for modifying said characteristic in a manner substantially corresponding to rotation thereof upwardly at high frequencies and downwardly at low frequencies about a substantially constantly maintained intermediate frequency point there- 4. The combination according to claim 3, further including means controlled by said ad-= justable means for increasing the duration of high frequency oscillation components simultaneously with said modification of the transmission-frequency characteristic.

5. In a system including a source of pluralfrequency sound-representing electric oscillations and means responsive to said oscillations: an oscillation-transmitting circuit interposed between said source and said responsive means;

means includedin said circuit for attenuating lower frequency components of said oscillations in a degree increasing with lowering frequency; means included in said circuit for amplifying higher frequency components of said oscillations in a degree increasing with increasing frequency;

and a single means for simultaneously adjusting said attenuation and said amplification, to increase or to decrease both simultaneously while maintaining substantially'constant the amplitude of. intermediate frequency components.

6. In a system including a source of plural-frequency sound-representing electric oscillations and means responsive to said oscillations: a circult having normally a substantially flat transmission-frequency characteristic interposed between said source and said responsive means; and means included in said circuit for modifying said characteristic, comprising means for attenuating lower frequency components of said oscillations in a degree increasing with lowering frequency, means for amplifying higher frequency components of said oscillations in a degree increasing with increasing frequency, and a single means for simultaneously adjusting said attenuation and said amplification, to increase or to decrease both simultaneously while maintaining substantially constant the transmission of intermediate frequency oscillation components.

7. The combination according to claim 6, further including means controlled by said adjusting means for increasing the duration of high frequency oscillation com onents simultaneously with increase of said attenuation and of said amplification.

8. In a system including a sourceof plural frequency sound-representing electric oscillations and means characterized by a high input imped ance for receiving said oscillations and translating the same into sound: a tone modifying device interposed between said source and said means and comprising a resistance and an inductance in series with said resistance, said resistance and a portion only of said inductance being connected across said source, and said means being connected across said resistance and the entire said inductance.

9. In a system including a source of plural frequency sound-representing electric oscillations and means characterized by -a high input impedance for receiving said oscillations and translating the same into sound: a tone control device interposed between said source and said means and comprising a resistance and an inductance in series with said resistance, said resistance and a portion only of said inductance being connected across said source, said means being connected across said resistance and the entire said inductance, and said resistance being variable whereby to produce substantial variation of the transmission-frequency characteristic of said device within the range of audible frequencies.

10. In a system including a source of plural frequency sound-representing electric oscillations and means characterized by a high input impedance for receiving said oscillations and trans lating the same into sound: a tone modifying device interposed between said source and said means and comprising a step-up transformer having primary and secondary windings and further comprising a single resistance in series with both of said windings, said resistanceand said primary winding being connected across said sources, and said means being connected across said resistance and said secondary winding.

11. In a system including a source of plural frequency sound-representing electric oscillations and means characterized by a high input impedance for receiving said oscillations and translating the same into sound: a tone control device interposed between said source and said means and comprising a step-up transformer having primary and secondary windings and fura,oss,aos

ther comprising a single resistance in series with.

both of said windings, said resistance and said primary winding being connected across said source, said means being connected acrosssaid resistance and said secondary winding, and said resistance being variable whereby to produce substantial variation of the transmissionqfrequency characteristic of said device within the range of audible frequencies.

12. The combination with an oscillation transmitting circuit, a voltage-responsive device connected across the output of said circuit, andan oscillation source for applying plural frequency sound representing electric oscillations across the input of said circuit, of means included in said circuit for stepping up in voltage a fractional part of each different frequency component of said applied oscillations, such fractional part of' each component being greater the higher the frequency of that component; means included in said circuit for transmitting without step-up the remaining fractional part of each such different frequency component; and means for simultaneously varying all of said fractional parts and the input impedance of said circuit at lower audible frequencies, to increase said fractional parts while decreasing said impedance and vice versa.

" 13. In a system including a source of plural frequency sound-representing electric oscillations and means characterized by a high input impedance for receiving said oscillations and translating the same into sound: a tone modifying device interposed between said source and said device and comprising a resistance, an inductance in series with said resistance, and capacity associated with said inductance to form a closed resonant circuit, said resistance and a portion of said inductance being connected across said source, and said means being connected across said resistance and a larger portion of said inductance.

14. In a system including a source of plural frequency sound-representing electric oscillations and means characterized by a high input impedance for receiving said oscillationsv and translating the same into sound: a tone control device comprising a resistance, an inductance in series with said resistance, and capacity associated with said inductance to form a closed resonant circuit, said resistance and a portion of said inductance being connected across said source, said means being connected across said resistance and a larger portion of said inductance, and said resistance being variable whereby to produce substantial variation both of the transmission-frequency characteristic of said circuit within the audible frequency range and of the amplitude of oscillations impressed on said resonant circuit.

15. In a system including a source of plural frequency sound-representing electric oscillations and means characterized by a high input impedance for receiving said oscillations and translating the same into sound: a tone modifying device interposed between said source and said means and comprising a step-up transformer having primary and secondary windings, further comprising a single resistance in series with both of said windings, and further comprising capacity associated with said secondary winding to form a closed resonant circuit, said resistance and said primary winding being connected across said source, and said means being connected across said resistance and said secondary winding.

16. In a system including a source of plural frequency sound-representing electric oscillations and means characterized by a high input impedance for receiving said oscillations and translating the same into sound: a tone control device interposed between said source and said means and comprising a step up transformer having primary and secondary windings, further comprising a single resistance in series with both said windings, and further comprising capacity associated with said secondary winding to form a closed resonant circuit, said resistance and said primary winding being connected across said source, said means being connected across said resistance and said secondary winding, and said resistance being variable whereby to produce substantial variation both of the transmissionfrequency characteristic of said circuit within the audible frequency range and of the amplitude of oscillations impressed on said resonant circult.

17. In a system including a source of plural frequency sound-representing electric oscilla-.

tions, said source having output impedance Ru, and means characterized by a high input impedance ior receiving said oscillations and translating the same into sound: a tone control device interposed between said source and said means and comprising a transformer having a primary of inductance L anda secondary having a turn ratio T to the primary and further comprising a resistance R in series with both said primary and said secondary. the eflicincy of 7 transmission of said oscillations by said device from said source to said means beinggiven substantially by the expression:

the value of T being greater than unity. the value of R0 being large compared to that of L at low frequencies, and the value of R being variable from a relatively low value to a value large compared both to that of Bo and to that of uL at high frequencies.

CHARLES T. JACOBS. 

